1. Field of the Invention
The present invention relates to a semiconductor laser device having a nitride semiconductor crystal.
2. Description of the Related Art
A semiconductor laser device having a nitride semiconductor crystal is a laser device which oscillates within a wavelength band around a blue wavelength. It has been confirmed that such a laser device has a laser oscillation function.
Many semiconductor laser devices which have been confirmed to provide laser oscillation have a gain guiding structure or a simple ridge structure with no buried structure. For these semiconductor laser devices, there is a demand to improve the driving efficiency and to provide a waveguide which allows for a stable single transverse mode.
In the conventional ridge type semiconductor device, a GaN-type material is typically cleaved along the {1, xe2x88x921, 0, 0} facet thereof to provide a cavity surface. This is because the {1, xe2x88x921, 0, 0} facet of a GaN-type material provides a desirable cleaved surface.
A semiconductor laser device having a buried structure is known in the art as a type of semiconductor laser device which has both a desirable current constricting function and an optical waveguide characteristic with a low propagation loss.
A conventional semiconductor laser device having a buried structure will be described below with reference to FIG. 3. Referring to FIG. 3, a buffer layer 2 of i-GaN and an n-side contact layer 3 of n-GaN are provided in this order on a substrate 1. An n-side cladding layer 4 of n-Al0.1Ga0.9N is provided on a portion of the n-side contact layer 3. A light emitting layer 5 having a multilayer structure, a first p-side cladding layer 6 of p-Al0.1Ga0.9N, and an n-type current blocking layer 7 of n-Al0.2Ga0.8N are provided in this order on-the n-side cladding layer 4. The n-type current blocking layer 7 includes a stripe groove 7a which reaches the first p-side cladding layer 6. The stripe groove 7a is provided to localize the electric current passing therethrough to a narrow area. A second p-side cladding layer 8 of p-Al0.1Ga0.9N is provided on the n-type current blocking layer 7 so as to fill up the groove 7a. A p-side contact layer 9 of p-GaN and a p-side electrode 10 are provided in this order on the second p-side cladding layer 8.
An n-side electrode 11 is provided on the other portion of the n-side contact layer 3.
If such a buried type semiconductor device has its cleaved surface along the {1, xe2x88x921, 0, 0} facet, as in the ridge type semiconductor device, the stripe groove is formed along the  less than 1, xe2x88x921, 0, 0 greater than  direction of the n-type current blocking layer 7.
FIG. 4 is a diagram illustrating a portion of an intermediate structure of the conventional semiconductor laser device during the crystal growth of the second p-side cladding layer 8. As illustrated in FIG. 4, the second p-side cladding layer 8 being grown contains a large number of disturbed surfaces along the stripe groove 7a. 
In such a conventional semiconductor laser device, the crystal quality of the second p-side cladding layer 8 is quite poor along or near the stripe groove 7a. Such a semiconductor laser device has a substantial propagation loss. However, the poor crystal quality has not been addressed as a problem, or it has been neglected. The present inventors have found that each of the disturbed surfaces formed along the stripe groove 7a extends along the (1, xe2x88x921, 0, 1) facet and at a certain angle with respect to the  less than 1, xe2x88x921, 0, 0 greater than  direction of the stripe groove.
According to one aspect of this invention, a semiconductor light emitting device includes: a substrate; a light emitting layer; a semiconductor layer of a hexagonal first III-group nitride crystal; and a cladding layer of a second III-group nitride crystal. A stripe groove is provided in the semiconductor layer along a  less than 1, 1, xe2x88x922, 0 greater than  direction.
Thus, the stripe groove is oriented along the  less than 1, 1, xe2x88x922, 0 greater than  direction, so that the crystal growth surface (1, xe2x88x921, 0, 1) and the stripe groove are parallel to each other. As a result, a crystal grows along a surface which is substantially parallel to the side surface of the  less than 1, 1, xe2x88x922, 0 greater than  stripe groove. Thus, it is possible to prevent disturbed surfaces from being produced in the cladding layer, thereby improving the crystal quality in the vicinity of the groove.
In one embodiment of the invention, a slope of the stripe groove extends at an angle of about 20xc2x0 to about 80xc2x0 with respect to a primary surface of the substrate.
With such a structure, as the angle between the slope of the groove and the primary surface of the substrate is about 20xc2x0 or more, it is possible to reduce the area of the semiconductor layer existing below the slope where the thickness of the semiconductor layer is small, and to suppress the current flowing through the semiconductor layer. Moreover, as the angle between the slope of the groove and the primary surface of the substrate is about 80xc2x0 or less, it is possible to suppress the shift or variation in the composition of the III-group nitride material in the semiconductor layer or in the cladding layer.
In one embodiment of the invention, an electric resistivity of the semiconductor layer is larger than that of the cladding layer.
With such a structure, the resistance of the semiconductor layer can be increased, so that the semiconductor layer may function as a current blocking layer.
In one embodiment of the invention, a conductivity type of the semiconductor layer is opposite to that of the cladding layer.
With such a structure, as the conductivity type of the semiconductor layer is opposite to that of the cladding layer, the semiconductor layer may function as a current blocking layer.
In one embodiment of the invention, a refractive index of the semiconductor layer is smaller than that of the cladding layer.
With such a structure, it is possible to obtain a semiconductor light emitting device which realizes a single transverse mode current constricting function.
In one embodiment of the invention, the semiconductor layer includes AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61), and the cladding layer includes AlyGa1xe2x88x92yN (0xe2x89xa6yxe2x89xa61 and y less than x).
With such a structure, as the refractive index of the semiconductor layer is smaller than that of the cladding layer, it is possible to obtain a semiconductor light emitting device which realizes a single transverse mode current constricting function.
In one embodiment of the invention, the cladding layer includes a groove which extends along a (1, xe2x88x921, 0, 1) facet.
With such a structure, it is possible to maintain a desirable crystal growth surface of the cladding layer, thereby improving the crystallinity of the cladding layer.
In one embodiment of the invention, the stripe groove is formed by an isotropic dry etching process.
With such a-structure, it is possible to obtain a semiconductor layer having a desirable crystal growth surface, thereby improving the crystallinity of the cladding layer.
According to another aspect of this invention, a method for producing a semiconductor light emitting device is provided. The method includes the steps of: providing a light emitting layer; providing a semiconductor layer of hexagonal first III-group nitride crystal; providing a stripe groove in the semiconductor layer along a  less than 1, 1, xe2x88x922, 0 greater than  direction by an isotropic dry etching process; and providing a cladding layer of a second III-group nitride crystal on the semiconductor layer after the step of providing the stripe groove.
With such a structure, as the stripe groove is provided in the semiconductor layer along the  less than 1, 1, xe2x88x922, 0 greater than  direction by an isotropic dry etching process, the slope of the groove in the semiconductor layer can be a desirable crystal growth surface.
In one embodiment of the invention, in the step of providing the stripe groove, a slope of the stripe groove extends at an angle of about 20xc2x0 to about 80xc2x0 with respect to a primary surface of a substrate.
With such a structure, as the angle between the slope and the primary surface of the substrate is about 20xc2x0 or more, it is possible to reduce the area of the semiconductor layer where the thickness thereof is small, and the semiconductor layer can be a current blocking layer which allows less current to pass therethrough. Moreover, as the angle between the slope of the groove and the primary surface of the substrate is about 80xc2x0 or less, it is possible to suppress the shift or variation in the composition of the III-group nitride material in the vicinity of the slope, which may occur during a crystal growth process.
In one embodiment of the invention, the cladding layer is provided by a crystal growth method such that a crystal growth rate is greater on the stripe groove than on a surface other than the stripe groove.
With such a structure, the slope of the semiconductor layer can be an even more desirable crystal growth surface.
In one embodiment of the invention, the isotropic dry etching process used in the step of providing the stripe groove is a reactive ion etching process with a mixed gas containing boron chloride and nitrogen.
With such a structure, the slope of the semiconductor layer can be an even more desirable crystal growth surface.
In one embodiment of the invention, the step of providing the stripe groove includes the steps of: providing a mask having a stripe opening along a  less than 1, 1, xe2x88x922, 0 greater than  direction on the semiconductor layer; providing a stripe groove in the semiconductor layer by using the isotropic dry etching process after providing the mask; and removing the mask after the step of providing the stripe groove.
With such a structure, as the mask having the stripe opening along the  less than 1, 1, xe2x88x922, 0 greater than  direction is provided on the semiconductor layer, and the groove is provided in the semiconductor layer by using the isotropic dry etching process, it is possible to provide the groove having a desirable crystal growth surface in the semiconductor layer.
In one embodiment of the invention, a resist mask is used as the mask.
With such a structure, the angle between the crystal growth surface and the primary surface of the substrate can be controlled so as to provide a groove having a desirable crystal growth surface in the semiconductor layer.
Thus, the invention described herein makes possible the advantages of: (1) providing a semiconductor laser device with a reduced propagation loss; and (2) providing a method for producing such a semiconductor laser device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.